Offset mold twinning

ABSTRACT

Thermoplastic parts are made of predominantly single unit thickness but with marginal areas of increased thickness by molding two parts in a common mold having overlapping mold portions. Two thicknesses of thermoplastic material are molded together in the areas of increased thickness at opposite ends of the common mold and the parts are separated from one another by trimming such that each part has an area of increased solid wall thickness and an area of reduced thickness.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. patent application Ser. No.60/733,644, filed Nov. 4, 2005 and titled “Offset Mold Twinning”.

STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

This invention relates to thermoforming, and in particular, to a methodof creating a thermoplastic part with a band of selectively increasedwall thickness or double wall features along one or two edges of thepart.

BACKGROUND OF THE INVENTION

There is a need on many differing types of plastic parts when only onearea of the part has to have substantial wall thickness and theremainder of the part can be thinner. One example of this is the plasticlid on a trash dumpster. The main requirement of a lid on a dumpster isto keep rain and snow out and contain the trash. A lid can be made usingvery thin material molded with structural ribs that will satisfy bothrequirements. When such a lid is made of thin material, it typicallyfails by tearing around the pivot point where the lid is hinged to thecontainer. The most typical method of hinging is by placing a steel rodthrough holes that are drilled in the ribs or knuckles that are formedin the plastic. The existing technology is that lids are now typicallymade of thicker material to toughen the hinge area, resulting in ahigher cost.

A second example of this need for selectively increased wall thicknessis a wall panel of a plastic shelter like a portable animal house forcalves or any similar application. The upper portions of the wall areprimarily for weather protection. Nearer to the bottom the walls aresubject to considerable abuse and need to be stronger and thicker tofacilitate fastening to the base, that is often times of anothermaterial like wood or solid plastic structural material. Additionally itmay be necessary to have the wall material surround a majority of thebase. Heretofore such a wall panel would be molded of thicker materialin its entirety or constructed using a twinsbeet thermoforming method,at additional cost.

There are numerous other applications also within this category. Forexample, such applications include liners in semi trailers where thebottom takes a majority of the abuse and the sidewalls of materialhandling containers to facilitate connecting the panels to the base.

Parts that are made using an injection, compression or casting moldingprocess require an inner and outer mold that is completely filled withthe plastic material. Any requisite additional thickness is accomplishedby increasing the distance between the inner and outer mold halves andcompletely filling the space. Parts that are made by thermoforming orblow molding are limited by the starting stock or sheet thickness,typically with pressure or a vacuum on at least one side of the sheet orstock. The subject of this patent is to provide increased stockthickness when using the thermoforming or blow molding process.

Simple common thermoforming utilizes a single sheet of plastic that isheated then molded utilizing vacuum or pressure or both to draw thematerial into a one sided mold. Once formed, one side of the plastic isagainst the mold and the other side is open to the atmosphere forcooling. Typically this process is performed on a thermoforming machineof which there are many sizes and configurations.

There also exists a well-known thermoforming process called twin sheetmolding. This processing method yields a finished part that is doublewalled, with inner and outer walls, with varying amounts of spacebetween them. The two sheets of thermo plastic are married in themolding machine and typically fused together at particular areas in aninvisible seam completely, or nearly so, around the perimeter and invarious other areas advantageous to the application. Twinsheetthermoforming requires two sheets of plastic heated simultaneously andtwo molds mounted face to face in the molding machine. Typicallytwinsheeting is performed on a specially configured thermoformingmachine of which there are many sizes. Any shape hollow part can betwinsheeted but the most typical application is large rectangular partswith only several inches or less separating the walls. Other processesare better suited for rounder more voluminous parts like tanks. Theadvantages of twinsheet thermoforming are numerous, the most importantbeing the ability to make a hollow part that has considerable structuralstrength and the ability to have differing features on the top andbottom or opposite sides of the part.

Blow molding is a process that shares the same molding system asthermoforming, but the stock in the shape of a tube, called the parison,is extruded into a vertical standing open mold. When large flat partsare molded, a parison stretcher is employed. This mechanism essentiallystretches the parison to extend beyond the edges of the mold. Theextrusion process provides the heat for the blow molding process. Whenmaking a simple flat panel by blowmolding the mold is configured toprovide two parts, face-to-face.

All three of the methods, simple and twinsheet thermoforming andblowmolding, require that the molded part be removed from the scrapmaterial around its perimeter. This can be accomplished in many fashionsbut the preferable way when trimming parts is to rout them.

To enable the routing or other cutting of the parts from the scrap andto separate them, the molded parts must be shaped such that the routerbit can separate the material but not cut through the primary part.Additionally, two primary parts as molded must have adequate distancebetween them to allow access for the trimming equipment. The preferableway to accomplish this is to have the material that creates a twinnedband depart from the primary part at a 45 degree angle and then move ata steeper angle away until it becomes the primary part from the oppositecavity—thus providing the tool access space. The steeper angle minimizesthe overall length of the tooling, saving cost of the raw material andthe tooling.

SUMMARY OF THE INVENTION

The invention is a method of simultaneously molding multiple plasticparts and includes providing a first mold portion and providing a secondmold portion facing the first mold portion. The first and second moldportions are offset such that at one or more edges of each of two of themolded parts to be produced by the mold portions an increased thicknessarea of each part is produced adjacent to a reduced thickness area ofeach part. Plastic material between the mold portions is molded bypressing two layers of plastic together in the increased thickness areasof each part with each layer forming the reduced thickness area of oneof the parts so as to produce the two molded parts connected together byplastic material. The molded parts are separated from one another bycutting the plastic material connecting the parts together so that eachmolded part includes at least one area of increased thickness and atleast one area of reduced thickness.

Objects and advantages of the invention will appear in the detaileddescription and claims which follow. In the description, reference ismade to the accompanying drawings which illustrate a preferredembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross sectional view of two mold portions moldingtwo parts (dumpster lids) according to the present invention;

FIG. 1B is an enlarged view of one end of the still connected dumpsterlids of FIG. 1A which shows the area of increased thickness on one endof part 10A;

FIG. 1C is a cross sectional view similar to FIG. 1B of the stillconnected dumpster lids of FIG. 1A after being demolded and illustratingwhere the composite part could be cut to separate the two parts;

FIG. 2A is a perspective view of a dumpster lid made according to theinvention as illustrated in FIG. 1A;

FIG. 2B is a detail cross-sectional view of the lid of FIG. 2A;

FIG. 3A is a perspective view of the dumpster lid of FIG. 2A and showinga cutaway portion;

FIG. 3B is a detail view of the cutaway portion of FIG. 3A;

FIG. 4A is a perspective view of an animal shelter made using a methodof the invention; and

FIG. 4B is a cross-sectional detail view of the lower portion of theshelter of FIG. 4A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a combination of single sheet andtwinsheet thermoforming. The same method works with blowmolding. In theexamples described, the parts that are formed are primarily single sheetwith a band along one edge that needs to be thicker or twinsheet molded.

The present invention is practiced in one way by mounting tworeconfigured single sheet style molds face to face in the thermoformingmachine to form two parts. Each of the molds consists of two sections:the primary section that molds one surface of the complete part and thetwin section that forms the second surface of the selectively twinnedsection of the opposite part. The molds are mounted with the primarysections opposite and offset the requisite amount to provide space forthe twinning section to mold the twin section of the opposite part. Theshape of the twinning section of the mold is such to provide the shapeand thickness of the selective twinned portion of the part.

In particular, referring to FIG. 1A, two mold halves 14A, 14B arearranged face to face in the thermoforming machine to form two dumpsterlids 10A, 10B having respective hinge areas 12A and 12B. A finisheddumpster lid 10 with hinge area 12 is illustrated in FIGS. 2A and 3A.The two lids 10A, 10B are formed as a composite part but are separatedafter molding. Each mold half 14A, 14B includes a respective first moldportion corresponding to an outer surface of the lid to be formed bythat mold half and a respective second mold portion corresponding to thedouble thickness band of the other finished lid. The length of eachfinished part is indicated by Finished Part A and Finished Part B. Eachfinished lid will include a portion having a single sheet (reduced)thickness and a portion having a double sheet (increased) thickness. InFIG. 1A, the length of the band of double thickness, i.e., the selectivetwinning, is indicated for each part by BAND A and BAND B respectively.

FIG. 1B shows an enlarged view of one end of the still connecteddumpster lids of FIG. 1A and better illustrates the area of increasedthickness on one end of what will become finished lid 10A. Forillustration purposes, the sheet material 11A is delineated from thesheet material 11B in the increased thickness portion.

Referring to FIGS. 2A-2B and 3A-3B, in this example of dumpster lid 10,the finished lid 10 might be 6′ long by 4′ wide—a typical real size ofmany different industry sizes. As shown in FIG. 2A, in the finished lid10, hinge area 12 runs along one 4′ edge. Further, the finished lid 10includes parallel ribs 16 running lengthwise, with ridges 17 between theribs 16 extending widthwise. The hinge is created by drilling holes 18(FIG. 3A) in the side surfaces of the ribs near one end of the lid so asteel hinge pin (not shown) can be inserted parallel to the widthdimension. The end that will be hinged is slightly different in shapethan the opposite end to accommodate the hinging motion. When this newoffset twinsheet process is used, the area of the ribs 16 that aredrilled for the hinge is molded double thick along the entire width ofthe lid and extending from the hinge end into the part for severalinches, for example three inches. This will require the mold portions tobe offset by the required three inches of overlap plus several moreinches to accommodate stretching of the sheet and trimming requirements.In this example the required double thick area of 3 inches by 4 feet,the two sheets 11A, 11B of plastic are completely fused together with nospace between the two walls. This is accomplished by having the two moldportions closely match with only the space between them equal to thecombined thickness of the formed double sheets of plastic. Relief areasare provided to allow a place for excess material to flow to ifnecessary or if advantageous to the design—like the area around thehinge pin holes in this example.

As mentioned, the two lids 10A, 10B are formed as a composite part andare subsequently separated from each other. In particular, as indicatedin the detailed view of FIG. 1C, a router is shown to indicate theangled wall area where the lid 10A could be separated from the lid 10B.Other cut locations for separating the parts are also possible, withsubsequent trimming to be performed if necessary. It should beunderstood that a similar cut would be made on the other end as well,with the cuts being made after the composite part is ejected from themolds. These cuts are made on the inner side of the double thicknessarea that is retained in the respective lid 10A or 10B. There would alsobe one or more cuts, indicated by 23, made along the outside edge of theincreased thickness portions to remove the scrap material 22A and 22B.Again, additional trimming may be necessary depending on the location offirst cuts and the desired finished configuration of each part.

In the example of FIGS. 4A and 4B, a shelter 50 is illustrated. In thiscase the selective double wall requirement results in a significantamount of the selectively twinned band area of increased thickness tohave a space between two walls 58, 59 of plastic, in which a wood orcomposite base 60 is positioned and assembled to the plastic walls 58,59 by a bolt or other fastener (not shown). The selectively twinnedband, having a length indicated as BAND C, is formed in the same manneras described above, but trimmed at a different location as illustratedin FIG. 4B. The entire shelter 50, which might be 8-feet by 8-feet basedimensions and 6 feet high and consist of four wall panels 52, one ofwhich would be configured so as to provide an entry way 54, and a roof56. To provide for easy assembly, and a strong mounting system to mountthe walls to the structural base, the walls could have the bottom bedouble walled in a band of selective twinsheeting, with the band havinga length of around a few inches to perhaps a foot or so, depending onthe dimensions of the structural base member 60. Unlike the completelyfused band in the dumpster lid 10, for this application a space or gapwould be provided between the walls 58, 59 of the twin band equal to thewidth of the structural base member 60, which may be for example wood ora composite material for structural rigidity. By having this lowerportion of each wall capturing the base on both the outer and innersides and cladding it from the top, the base is protected from theclimate and animal waste. Also, if the side of the shelter 50 isimpacted from either the inside or outside of the shelter, the force ofthe impact is born by the panel 52 rather than the fasteners.

To be cost effective, it is preferred that two identical or nearly soitems be molded using the process of this invention. Typically both theparts would be molded using the same starting gauge material; theresulting area of selective twinsheeting would be identical on bothparts and effectively double thick. The plastic sheet that is used willbe larger than the size that would ordinarily be used to make the partby roughly the amount of selective twinsheeting.

Another variation of this process includes offsetting the two molds intwo directions so that two adjacent edges of a part can be selectivelytwinsheeted.

It is anticipated that often an existing set of single sheet molds couldbe reconfigured and a twinning section added to utilize a method of theinvention.

Preferred embodiments of the invention have been described inconsiderable detail. Many modifications and variations to theseembodiments will be apparent to a person of ordinary skill in the art.Therefore, the invention should not be limited to the embodimentsdescribed herein.

1. A method of simultaneously molding multiple plastic parts,comprising: providing a first mold portion; providing a second moldportion facing the first mold portion; offsetting the first and secondmold portions such that at at least one edge of each of two of themolded parts to be produced by said mold portions an increased thicknessarea of each part is produced adjacent to a reduced thickness area ofeach part; molding plastic material between said mold portions bypressing two layers of plastic together in the increased thickness areasof each part with each layer forming the reduced thickness area of oneof the parts so as to produce the two molded parts connected together byplastic material; and separating the molded parts from one another bycutting the plastic material connecting the parts together so that eachmolded part includes at least one area of increased thickness and atleast one area of reduced thickness.
 2. A method as claimed in claim 1,wherein the material cut to separate the molded parts includes materialin the area of reduced thickness.
 3. A method as claimed in claim 1,wherein the material cut to separate the molded parts includes materialthat is adjacent to the area of increased thickness of at least one ofthe parts.
 4. A method as claimed in claim 1, wherein the areas ofincreased thickness are along one edge of each molded part.
 5. A methodas claimed in claim 1, wherein an area of increased thickness is alongtwo edges of each molded part.
 6. A method as claimed in claim 5,wherein the two edges of each molded part are adjacent to one another.7. A method as claimed in claim 1, wherein the molding method is athermoforming molding method.
 8. A method as claimed in claim 1, whereinthe molding method is a blowmolding molding method.
 9. A method asclaimed in claim 1, wherein the area of increased thickness includes atleast one area in which two layers of plastic are separated by a gap.10. A method as claimed in claim 1, wherein the area of increasedthickness includes at least one area of two layers of plastic fusedtogether.
 11. A method as claimed in claim 1, wherein the mold portionsare offset from one another so as to provide an access area in which toseparate the parts by cutting.
 12. A method as claimed in claim 1,wherein the mold portions create an angled wall of plastic material inthe area where the parts are separated by cutting, the angled wall beingaccessible to be cut so as to separate the parts from one another.
 13. Amethod as claimed in claim 1, wherein the molded parts are essentiallyidentical in size and shape.